Gypsum set accelerator

ABSTRACT

A gypsum set accelerator, produced by compression milling a mixture of ground gypsum rock and starch, and the addition thereof to an aqueous slurry of settable calcined gypsum such as in the forming of paper-covered gypsum wallboard.

United States Patent 1191 Burkard et al.

1451 Nov. 18, 1975 GYPSUM SET ACCELERATOR [75] Inventors: Edward A. Burkard, East Amherst; Donald A. Kossuth, Buffalo, both of [73] Assignee: National Gypsum Company,

Buffalo, NY.

[22] Filed: Oct. 15, 1974 [21] App]. No.: 514,455

Related US. Application Data [62] Division of Ser. No. 358,191, May 7, 1973, Pat. No.

[52] US. Cl. 106/114; 106/115; 106/315 [51] Int. Cl. C04B 11/22 [58] Field of Search 106/114, 115, 314, 315

Primary Examiner-.l. Poer Attorney, Agent, or Firm-Robert F. Hause [57] ABSTRACT A gypsum set accelerator, produced by compression milling a mixture of ground gypsum rock and starch, and the addition thereof to an aqueous slurry of settable calcined gypsum such as in the forming of papercovered gypsum wallboard.

4 Claims, 1 Drawing Figure US. Patent Nov. 18, 1975 3,920,465

CCISO4'2H2O Starch Boll Mill To |5,000 cm. /gm.

Foaming CoSO -l/2H O Paper Pulp Wflier Agent Gypsum Wollbodrd Core GYPSUM SET ACCELERATOR This application is a division of our copending application, Ser. No. 358,191, filed May 7', 1973, now US. Pat. No. 3,870,538, issued March 11, 1975.

This invention relates to a novel gypsum-starch composition for addition to a calcined gypsum slurry to accelerate the setting of the gypsum, and is particularly directed to an-.improved method of making gypsum wallboard.

Calcined gypsum, mixed with a proper amount of water, will normally set in about 25 minutes to a hardness such that a /4. lb. Gilmore needle will no longer make any substantial indentation, referred to as initial set, and in about 35 minutes to a hardnesssuch that a 1 lb. Gilmore needle will no longer make any substantial indentation, referred to as final set.

Two of the most'common uses of calcined gypsum are (l) in forming the core of the well-known papercovered gypsum wallboards and (2) in forming the relatively thick wall surfacing material in a plastered wall. In making gypsum wallboard it has been common practice to add a set accelerating agent in an amount sufficient to permit the wallboard core to reach initial set in about 3 to 4 minutes, and to reach final set in about 5 to 8 minutes. in preparing bags of calcined gypsum for job site mixture with water for plastering walls it has been common to add set retarders and set stabilizers, which include accelerators, such that when admixed water and aggregate the final set of the plaster mixture is reached in about an hour to 2 hours.

The gypsum setting reaction involves the reaction of calcium sulfate hemi-hydrate and water to form calcium sulfate dihydrate. A most common form of accelerator for this reaction is calcium sulfate dihydrate. There are many forms of calcium sulfate dihydrate which can be used and many combinations of ingredients with which it is combined, when used as a set accelerator, depending on which of several alternative characteristics are most important for a particular use.

In preparing bags of plaster for use on job sites for plastering walls, the calcined gypsum, which is the primary ingredient, is commonly fed to the bagging ma chine while it is still very warm from the calcining process. If a calcium sulfate dihydrate is added at this time, also, it is subject to being partially calcined, that is partially dehydrated, by the heat from the calcined gypsum, thus losing much of its accelerating ability. To prevent this calcining of the accelerator, it has been taught in US. Pat. 'No. 2,078,199 that a. general group of water-soluble carbohydrates known as sugar or a material intermediate between starch'and sugar, such as soluble dextrin, can be ball-milled together with the calcium sulfate dihydrate in a weight, ratio of 5-25: 100 and this water-soluble carbohydrate will function as a dehydration-preventing substance when the combination is used as an accelerator.

The present invention is directed to an improved accelerator for the set of gypsum wherein the primary novel characteristics have no relation to the dehydration prevention characteristics described above. The novel accelerator provides completely unexpected and marked advantages when used in the aqueous slurry of calcined gypsum for forming the core of paper-covered gypsum wallboard.

A common accelerator presently used in the manufacture of gypsum wallboard is fresh finely ground gypsum block. This finelyground block combined with calcined gypsum at a weight ratio of about 0.525: 100 will provide setting times suitable for wallboard manufacture, however it must be used substantially immediately after grinding, and, secondly, once the wallboard core has set, this ground block in effect dilutes the strength providing elements of the core, or to state it another way, it reduces the effective core purity.

. The accelerator of the present invention consists essentially of the compression-milled product of a mixture of calcium sulfate dihydrate and starch. In a preferred form, the novel accelerator retains its potency for many days and involves the usage of only about O .l:l00 parts of calcium sulfate dihydrate to calcined gypsum. There would also be involved the usage of 0.175: parts of starch to calcined gypsum. This starch, however, makes it possible to omit the separate addition of starch to the wallboard core formulation, which is an essential part of prior formulations, and functions through migration to the core to paper interface, to bond the paper to the core in the finished product. Furthermore, the total amount of starch required for providing bond of paper to core is less in accordance with the invention than with prior formulations.

It is an object of the invention to provide a novel accelerator for the setting reaction of calcined gypsum and water.

It is a further object of the invention to provide a novel method of making a gypsum accelerator and of making gypsum wallboard therewith.

It is a still further object of the invention to provide a novel wallboard formulation of substantially lower cost and improved quality.

These and other objects of the invention will be more readily apparent when considered in relation to the preferred embodiments as set forth in the specification and shown in the drawing in which there is a flow chart depicting the process of the present invention.

In accordance with the invention, CaSO .2H O, preferably in the form of crushed natural gypsum rock, is mixed with starch, preferably an inexpensive corn starch, and the mixture is subjected to the action of a ball mill for a time sufficient to cause what appears to be a mechanical combining of the two materials and sufficient to produce a final particle size of about 10,000 to 20,000 cm gm or more particularly from about 12,000 to 18,000 cm /gm, and preferably about 15,000 cm /gm, as measured by a Fisher Sub-Sieve Sizer. As shown in the drawing this ball-milled mixture is then mixed with the other ingredients of a gypsum wallboard core, including calcined gypsum, paper pulp, foaming agent and water, and the core is formed between facing papers. Of particular significance, it will be noted that the ball-milled mixture provides both the set accelerator and the core to paper binder in the'formulation. I

Calcium sulfate dihydrate has been known to be an accelerator, in its original form as mined and finely ground, known as land plaster, and also after it has been calcined and the rehydrated-as in block form. The

1 more expensive rehydrated block has been preferred due to its uniform shape and thus its ability to belun'iformly ground into a fine accelerator additive. Grinding gypsum rock with abrasive wheels does not produce a uniform rate of feed and it is essential that the plain ground land plaster be used'immediately after being ground to a fine powder.

In the present invention, land plaster can be ground to a fineness of about 93% through a 100 mesh screen and stored any convenient time period, and then be uniformly fed to a continuous feed ball mill along with 10 inches. The bond of the paper to the gypsum core was satisfactory in both examples.

In another example of the invention, 5.2 lbs. of a novel accelerator was used having 5% acid modified a continuous feed of starch which is obtained at a fine- 5 corn starch and 95% land plaster in place of the 5.2 lbs. ness of about 86% through a 100 mesh screen. Alternaof 1:1 ratio novel accelerator above. The setting times tively, the mixture can be ball milled in batches. The were satisfactory but a poor bond of paper to core was mixture, in accordance with the invention, can consist obtained. This is corrected for by adding additional of from about 1% to 99% calcium sulfate dihydrate and starch as a separate ingredient. Thus the novel accelerfrom about 99% to 1% starch, however the most unexator of this 5:95 ratio is a satisfactory accelerator in an pected benefits of the invention are obtained with mixunexpectedly lower total content but it lacks certain of tures of about 25% to 75% land plaster and about 75% the other unexpected advantages of the higher starch of 25% starch and particularly a ratio of about 1:1. One content accelerator.

of the primary advantages gained by the novel acceler- In another example of the invention, 5.2 lbs. of a ator lies in the unexpected economics provided, and novel accelerator was used made from 50% pearl the best economy is obtained by the use of land plaster starch and 50% land plaster, in the above-listed formuand raw corn starch as the raw materials. lation. Pearl starch is an unmodified raw starch. This Some examples of starch suitable with the invention formulation provided an initial setting time of 2 feet 30 are CPC lntemational (Corn Industrial Division) acid inches and a final setting time of 4 feet 45 inches, indimodified corn starch, Lauhoff Grain Company I-Ii Bond eating that 5.2 lbs. of this novel accelerator is more corn starch, A. E. Staley corn starch, Krause Milling than is needed.

Company grain starch, and Grain Processing Corpora- In the above formulas, the amount of water of hydration pearl starch. The forms of starch which will protion is set forth. An additional amount of water, on the vide the benefits of the invention include those starches order of 1000 lbs., which is used in making the slurry which, when dispersedwithin a wet, set gypsum core, from which the core is formed, which additional will migrate to the core to paper interface during drying amount is subsequently removed during drying of the and provide a bond of paper to core. wallboard, is not set forth in the formula. It should be In addition to the above method using a ball mill, a realized, however, that with the less solids in the Inrod mill can be used, or any other apparatus wherein vention formula than the Prior formula, proporthe mixed ingredients are subjected to a repeated comtionately less additional water, to be subsequently dried pression milling treatment. off, is required.

Following is an example of a prionstandard gypsum The starch content of 3.4 lbs. in the Prior formulawallboard core formulation and an example of a pretion should be recognized as a relatively low example of ferred embodiment of the present invention, in prior formulations, the range being generally from amounts suitable for making about 1000 sq. ft. of V2 about 3 lbs. to about 12 lbs., depending on many other inch thick paper-covered gypsum board. factors in the'formulation and process, and thus it will PRESENT PRIOR INVENTION C'alcined Gypsum 1491 lbs. 1491 lbs. Starch (added separately) 3.4 lbs. 0 Paper Pulp Fiber Reinforcement 11.9 lbs. 11.9 lbs. Ground Gypsum Block Accelerator 7.8 lbs. 0 Novel Accelerator (1:1 ratio) 0 5.2 lbs. Water of Hydration 140.6 lbs. 140.6 lbs. Soap (foaming agent) .3 lbs. .3 lbs. Core Total 1655 lbs. 1649 lbs. Front and Back Paper 144 lbs. 144 lbs. Board Weight 1799 lbs. 1793 lbs.

be further realized that the 2.6 lbs. of starch which pro- The above two formulas are given as examples both vides a suitable bond in the Invention formula is a of which will produce good quality /z inch wallboard of marked advantage. about 1800 lbs./M sq. ft. The 6 lbs. difference in weight Having completed a detailed disclosure of the preis of no importance. These two comparative formulas, ferred embodiments of our invention, so that others of slightly different total weight were used to best show may practice the same we contemplate that variations the dramatic differences in certain of the individual inmay be made without departing from the essence of the gredients in two otherwise similar formulas. All ingrediinvention. ents werekept constant except the starch and accelera- We claim: tor to demonstrate how the novel accelerator, made 1. A wallboard core formulation comprising a major from 2.6 lbs. of acid modified corn starch and 2.6 lbs. portion of calcium sulfate hemi-hydrate and a minor of land plaster, can perform as well as 3.4 lbs. of acid portion of fiber reinforcement, water of hydration, modified corn starch and 7.8 lbs. of ground block means for lowering the density and an accelerator for added separately. accelerating the setting reaction of calcium sulfate The Prior formula had an initial setting time of 3 feet 35 inches and a final setting time of 7 feet 35 inches. The Invention formula had an initial setting time of 3 feet 25 inches and a final setting time of 7 feet hemi-hydrate and water, said accelerator consisting essentially of the finely ground product of a compression milling of a mixture of about 1% to 99% calcium sulfate dihydrate and about 1% to 99% starch.

6 2. A wallboard core formulation as defined in claim I 4. A wallboard core formulation as defined in claim 2 wherein said mixture ratio is from about 1:3 to 3:1. wherein said accelerator has a surface area as deter- 3. A wallboard core formulation as defined in claim mined by a Fisher Sub-Sieve Sizer of from about 1, said formulation being free of any separate starch in- 5 12,000 to 18,000 cm lgm. gredient.

b CETEFEQATE OF CORRECTION iNvfNToRtS) 1 Edward A. Burkard and Donald A. Kossuth k Column 3, line 66, "feet" (first occurrence) should be iArr NT NO. 7 3, 920,465

UAW-U November 18, 1975 It is certified that error appears in the ab0verdentified patent and that said Letters Patent are hereby corrected as shown below.

Column 1, line 30, after "admixed" insert ---with---.

minutes--; "inches" should be --seconds--.

Column 3, line 66, "feet" (second occurrence) should be minutes--.

Column 3, line 67, "inches" should be ---seconds--.

Column 3, last line, "feet" (first occurrence) should be minutes---; "inches" should be -seconds---.

Column 3, last line, "feet" (second occurrence) should be --minutes---. Q

Column 4, first line, "inches" should be --seconds---.

Column 4, line 18, "feet" should be --minutes---.

Column 4, line 19, "inches" (first occurrence) should be ---seconds---.

Column 4, line 19, "feet" should be ---minutes--; "inches" (second occurrence) should be ---seconds---.

0 Signed and Scaled this.

thirtieth D f March 1976 [SEAL] Arrest. O

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uflarerrrs and Trademarks UNIIED e'r-ATrrs Pmlzhrr mum-i CERTIFICATE OF CORRECTION PArr NT NO. 3,920,465 UMED I November 18, 1975 INVENTO 1 Edward A. Burkard and Donald A. Kossuth It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 30, after "admixed" insert ---with---.

Column 3, line 66, "feet" (first occurrence) should be minutes---; "inches" should be --seconds--.

Column 3, line 66, "feet" (second occurrence) should be minutes--.

Column 3, line 67, "inches" should be ---seconds--.

Column 3, last line, "feet" (first occurrence) should be minutes---; "inches" should be ---seconds---.

Column 3, last line, "feet" (second occurrence) should be -minutes--.

Column 4, first line, "inches" should be --seconds---.

Column 4, line 18, "feet" should be ---minutes---.

Column 4, line 19, "inches" (first occurrence) should be ---seconds---.

Column 4, line 19, "feet" should be ---minutes---; "inches" (second occurrence) should be --seconds---.

Signed and Sealed this.

thirtieth D f March 1976 [SEAL] A ttes t:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (vmmisxinnvr nfPaIents and Trademarks 

1. A WALLBOARD CORE FORMULATION COMPRISING A MAJOR PORTION OF CALCIUM SULFATE HEMI-HYDRATE AND A MINOR PORTION OF FIBER REINFORCEMENT, WATER OF HYDRATION, MEANS FOR LOWERING THE DENSITY AND AN ACELARATOR FOR ACCELERATING THE SETTING REACTION OF CALCIUM SULFATE HEMI-HYDRATE AND WATER, SAID ACCELERATOR CONSISTING ESSENNTIALLY OF THE FINELY GROUND PRODUCT OF A COMPRESSION MILLING OF A MIXTURE OF ABOUT 1% TO 99% CALCIUM SULFATE DIHYDRATE AND ABOUT 1% TO 99% STARCH.
 2. A wallboard core formulation as defined in claim 1 wherein said mixture ratio is from about 1:3 to 3:1.
 3. A wallboard core formulation as defined in claim 1, said formulation being free of any separate starch ingredient.
 4. A wallboard core formulation as defined in claim 2 wherein said accelerator has a surface area as determined by a Fisher Sub-Sieve Sizer of from about 12,000 to 18,000 cm2/gm. 